Data transmission on lines and cables is subjected to disturbing influences such as, for example, electromagnetic fields that modify the information to be transmitted in that, for example, one or more bits are "turned around" during the transmission. Since it is particularly international long-distance calls that are susceptible to bit errors, international standardization committees (such as, for example, CCITT) have defined minimum rules for bit error rates. Optical transmission links are considerably less subject to external influences, so that the transmission reliability is extremely high given the employment of optical transmission links.
In general, every transmission link is composed of a total of two parts. One part relates to the transmission of information over a physical path between two nodes. These paths are being increasingly realized with optical message cables. The other part relates to the through-connection of the information in the respective nodes in which communication systems are installed.
The potential disturbing influence in the transmission-oriented part has been decisively reduced by employing optical transmission systems, whereas frequent errors in comparison thereto occur in the through-connection events in the switching networks of the communication systems.
As a consequence of their central significance for the communication systems, the switching networks are usually redundantly or even multiply implemented. This makes it possible to through-connect the information over a different switching network level given a malfunction of one switching network level. First, this ensues that one switching network level is in a quiescent condition while the other switching network level implements the current through-connection of the information; second, however, both switching network levels can also actively through-connect the corresponding information.
In order to be able to through-connect information over the respective switching network levels, the through-connect route in the switching network must be previously defined and set. This means that the information that arrives at the switching network and that is to be through-connected enters into the switching network via an input that is generally already determined, or that must be first determined before the through-connect event. This information leaves the switching network via an output that is also to be determined. The determination as to which input/output is employed in the through-connection of an information stream composed of a plurality of information words occurs by evaluating the signalling information with the central control platform of the communication system that defines and sets the through-connect route through the switching network. What is generally understood by a central control platform is the interplay of the central and decentralized control units of a communication system. The central control platform has stored therein the current data about the momentary occupation of the inputs/outputs of the entire switching network as well as the routes employed. The central control platform can therefore allocate as yet unoccupied input/outputs as well as corresponding through-connect routes through the switching network to further information streams to be through-connected. The central control platform makes this allocation at any time. A potential abort or the termination of an existing call is likewise implemented by the central control platform. This leads to the release of occupied inputs/outputs as well as used through-connect routes. Consequently, the data connected therewith must be constantly serviced by the central control platform, i.e. must be updated as needed. Since these are a matter of dynamically changing data, the servicing occurs in the memory of the central control platform. Storage on an external storage medium such as, for example, a hard disk is not possible because of the long access time connected therewith.
Sporadic and permanent errors can occur (bit error rate) upon through-connection of information across a switching network level. These are usually expressed therein that individual bits or a plurality of bits are falsified during the through-connect event, an inadmissible bit error rate arises or a false information stream is even output. Temporary or lasting false output can be the result. Sporadic errors originate in electromagnetic interference, in thermal influences or aging component parts, whereas permanently appearing errors result from defective or falsified holding memory areas, defective voice memories, defective addressing logics for holding and voice memories, from defective or superannuated component parts (for example lasers) or from a defective power supply.
In general, such errors are difficult to analyze and to eliminate insofar as they have no influence whatsoever on the function of the switching network as a through-connect unit. Prior art error recognition methods for recognizing and correcting bit errors in switching network have been realized, for example, in the form of on-line monitoring. German Published Application 24 27 668 discloses such a method. Additional check information is attached to the information words per channel that shape the information, being attached thereto before the actual through-connect event in the switching network. The check information, for example, can be a parity bit. What is thereby understood is the binary checksum over the individual bits of the corresponding information word. After the through-connect even, a binary checksum of the through-connected information word is again calculated (in the case of the parity bit) and the parity thereof with the transmitted parity bit is compared. When the two deviate from one another, then there is a transmission error that is retained in a corresponding table memory. Statements about the condition of the corresponding switching network level can thus be made over specific time spans. The switching network level that had the lowest bit error rate in the past can be used for the through-connection of the information streams based on these statistical statements.
German Patent Application P 41 28 412.7 (corresponding to U.S. Ser. No. 07/918,079 filed Jul. 24, 1992) also discloses a method for on-line monitoring and bit error reduction. A check information is calculated therein for the information words that form the information, being calculated in peripheral units before the through-connect event. This check information is appended to the respective information word. This is followed by a duplication of the information word supplemented by the check information, as well as by a through-connection over both switching network levels implemented following thereupon. After the through-connect event, the information words supplemented with the check information are subjected to another calculation of check information and the result is compared to the co-transmitted check information. Parallel thereto, the information words themselves are compared to one another bit-by-bit. Conclusions can then be drawn from the two evaluations as to whether or not transmission error is present or as to whether statements (although limited statements) can be made as to which bit was falsified. Corrections can then be implemented in this case. What is problematical about this procedure is that an unambiguous statement is not always possible. In this case, one has recourse to a statistical function f(s) that then forwards the information words to the destination subscriber that were connected via the switching network level that supplied the better statistical quality data in the past. In this case, however, there is a certain probability of the risk that the falsified information word will be forwarded to the destination subscriber.
Further, exact error localizing is only possible to a coarse scope in this method. Although statements can be made about which switching network level caused the transmission errors, a more detailed localization is not possible.
Over and above this, errors that effect a faulty through-connection in the switching network have serious consequences, i.e. that an information stream entering at a specific input of the switching network does not depart the switching network at the intended output but is forwarded to a random destination subscriber via an incorrect output due to a sporadic or permanently occurring error in the setting procedure. Such errors cannot be centerly discovered. Frequently, it is only the destination subscriber who recognizes the faulty through-connection.
Further, the fact that all dynamic setting data are deposited in the memory of the central control platform can lead to problems. For instance, in the case of a total outage of relevant parts of the central control platform, all setting data of the switching network as well as the data of the input/output occupation stored therein are thus lost. Relocation on to an external storage medium such as, for example, a hard disk is not possible since the setting data must be dynamically updated and the write/read procedures on the hard disk would use too much time, particularly in times of high traffic volume. The result of this is that all existing connections are aborted and the appertaining data memories must be initialized or must be updated in a complex way according to the hardware settings.